Enhanced inspection of extreme ultraviolet mask

ABSTRACT

The present invention discloses a method of increasing the contrast of an EUV mask at inspection by forming a multilayer mirror over a substrate; forming an absorber layer over the multilayer mirror; forming a top layer over the absorber layer; patterning the mask into a first region and a second region; and removing the top layer and the absorber layer in the first region.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of semiconductorintegrated circuit manufacturing, and more specifically, to a mask and amethod of fabricating a mask used in extreme ultraviolet (EUV)lithography.

[0003] 2. Discussion of Related Art

[0004] Ongoing improvements in lithography have allowed the shrinkage ofsemiconductor integrated circuits (IC) to produce devices with higherdensity and better performance. Deep ultraviolet (DUV) light with awavelength of 248, 193, 157, or 126 nanometers (nm) may be used foroptical lithography. However, a paradigm shift to Next GenerationLithography (NGL) should occur around the 70-nm node.

[0005] EUV lithography, a leading candidate -for NGL, is based onexposure with EUV light having a wavelength of 10-15 nanometers. EUVlight falls within a portion of the electromagnetic spectrum generallyknown as soft x-ray (2-50 nm). DUV lithography uses transmissive masksmade from fused quartz, but nearly all materials are highly absorbing atthe EUV wavelength so EUV lithography uses a reflective mask.

[0006] An EUV step-and-scan tool typically uses a 4×-reductionprojection system. A wafer is exposed by stepping fields across thewafer and scanning an arc-shaped region of the EUV mask for each field.An EUV step-and-scan tool may have a 0.10 Numerical Aperture (NA) with 4imaging mirrors. A critical dimension (CD) of 50-70 nm may be achievedwith a depth of focus (DOF) of about 1 micrometer.

[0007] Alternatively, an EUV step-and-scan tool may have a 0.25 NA with6 imaging mirrors to print a smaller CD of 20-30 nm, at the expense of asmaller DOF. Other tool designs with a 5×- or a 6×-reduction projectionsystem, may also be used for EUV lithography.

[0008] Optical inspection of a mask is based on a comparison of thelight signals in the patterned regions relative to the non-patternedregions. A high contrast is necessary in order to achieve sufficientsensitivity for defect detection. The transmissive masks used in DUVlithography can be inspected without difficulty since the contrastbetween the opaque regions and the clear regions is high at UV/DUVwavelengths. However, it is difficult to inspect the reflective masksused in EUV lithography since the contrast between the absorber regionand the mirror region is low at UV/DUV wavelengths.

[0009] Thus, what is needed is an EUV mask with high contrast at theinspection wavelength and a process for fabricating such an EUV mask.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1(a)-(e) are illustrations of a cross-sectional view of ahigh contrast EUV mask blank formed according to the present invention.

[0011]FIG. 2(a)-(d) are illustrations of a cross-sectional view of ahigh contrast EUV mask formed according to the present invention.

[0012]FIG. 3 is an illustration of a cross-sectional view of a highcontrast EUV mask of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0013] In the following description, numerous details, such as specificmaterials, dimensions, and processes, are set forth in order to providea thorough understanding of the present invention. However, one skilledin the art will realize that the invention may be practiced withoutthese particular details. In other instances, well-known semiconductorequipment and processes have not been described in particular detail soas to avoid obscuring the present invention.

[0014] Virtually all condensed materials absorb at the EUV wavelength soa mask for EUV lithography is reflective. A pattern on an EUV mask isdefined by selectively removing portions of an absorber layer to uncoveran underlying mirror. The inspection for defects on the ELV mask isusually done at UV/DUV wavelengths, but the contrast is ofteninadequate. The present invention is an EUV mask with high inspectioncontrast at the UV/DUV inspection wavelength and a process forfabricating such a high inspection contrast EUV mask.

[0015] Various embodiments of a process for fabricating a highinspection contrast EUV mask according to the present invention will bedescribed next. First, as shown in FIG. 1(a), a substrate 1100 with alow defect level and a smooth surface is used as the starting materialfor a high inspection contrast EUV mask of the present invention. It isdesirable to form the substrate 1100 out of a glass or glass-ceramicmaterial that has a low coefficient of thermal expansion (CTE). However,in some cases, the substrate 1100 may be formed from Silicon. AlthoughSilicon has a large CTE that may result in undesirable displacement ofprinted images, Silicon also has a high thermal conductivity and thusmay be a viable substrate as long as heat can be removed efficientlyfrom the mask during exposure.

[0016] Second, as shown in FIG. 1(b), a multilayer (ML) mirror 1200 isthen formed on the substrate 1100. The ML mirror 1200 has about 20-80pairs of alternating layers of a high index of refraction material 1210and a low index of refraction material 1220.

[0017] In one embodiment, the ML mirror 1200 has 40 pairs of alternatinglayers of a high index of refraction material 1210 and a low index ofrefraction material 1220. The high index of refraction material 1210 maybe about 2.8 nm thick Molybdenum (Mo) while the low index of refractionmaterial 1220 may be about 4.1 nm thick Silicon (Si). As needed, acapping layer 1230, such as about 11.0 nm thick Silicon (Si), may beformed at the top of the ML mirror 1200 to prevent oxidation ofMolybdenum in the environment. The ML mirror 1200 can achieve about60-75% reflectivity at the central illumination wavelength of about 13.4nm.

[0018] The ML mirror 1200 is formed over the substrate 1100 by using ionbeam deposition (IBD) or DC magnetron sputtering. The thicknessuniformity should be better than 0.8% across the substrate 1100. On theone hand, IBD results in less perturbation and fewer defects in theupper surface of the ML mirror 1200 because the deposition conditionscan usually be optimized to smooth over any defect on the substrate1100. On the other hand, DC magnetron sputtering is more conformal, thusproducing better thickness uniformity, but any defect on the substrate1100 will tend to propagate up through the alternating layers to theupper surface of the ML mirror 1200.

[0019] Third, as shown in FIG. 1(c), a buffer layer 1300 is formed overthe upper surface of the ML mirror 1200. The buffer layer 1300 may havea thickness of about 20-105 nm. The buffer layer 1300 may be formed fromSilicon Dioxide (SiO₂), such as low temperature oxide (LTO). A lowprocess temperature, typically less than about 150 C., is desirable toprevent interdiffusion of the alternating layers in the underlying MLmirror 1200. Other materials, such as Silicon Oxynitride (SiO_(x)N_(y))or Carbon (C) may also be used for the buffer layer 1300. The bufferlayer 1300 may be deposited by RF magnetron sputtering.

[0020] Fourth, as shown in FIG. 1(d), an absorber layer 1400 is formedover the buffer layer 1300. The absorber layer 1400 may be formed fromabout 45-215 nm of a material that will attenuate EUV light, remainstable during exposure to EUV light, and be compatible with the maskfabrication process. The absorber layer 1400 may be deposited with DCmagnetron sputtering.

[0021] Various metals, alloys, and ceramics may be used to form theabsorber layer 1400. Ceramics are compounds formed from metals andnonmetals. Examples of metals include Aluminum (Al), Aluminum-Copper(AlCu), Chromium (Cr), Nickel (Ni), Tantalum (Ta), Titanium (Ti), andTungsten (W). In some cases, the absorber layer 1400 may be partially orentirely formed out of borides, carbides, nitrides, oxides, phosphides,silcides, or sulfides of certain metals. Examples include NickelSilicide (NiSi), Tantalum Boride (TaB), Tantalum Germanium (TaGe),Tantalum Nitride (TaN), Tantalum Silicide (TaSi), Tantalum SiliconNitride (TaSiN), and Titanium Nitride (TiN).

[0022] Fifth, as shown in FIG. 1(e), a top layer 1500 is formed over theabsorber layer 1400. The top layer 1500 is usually thinner than theabsorber layer 1400. For example, the top layer 1500 may be about 20 nmthick.

[0023] In one embodiment, the top layer 1500 has higher absorbance thanthe absorber layer 1400. In another embodiment, the top layer 1500 haslower reflectivity than the absorber layer 1400. In still anotherembodiment, the top layer 1500 has both higher absorbance and lowerreflectivity than the absorber layer 1400.

[0024] Instead of forming the top layer 1500 as a discrete layer overthe absorber layer 1400, another embodiment of the present inventioncontemplates forming the top layer 1500 from an original surface of theabsorber layer 1400. In the case where the absorber layer 1400 is formedfrom Tantalum Nitride (TaN), the top layer 1500 may be formed byincorporating Fluorine (F) at the upper surface of the Tantalum Nitride.

[0025] The top layer 1500 may be formed by treating the absorber layer1400 with a Perfluorocompound (PFC), such as C_(x)H_(y)F_(z) gas, with acertain plasma power. The chamber may be similar to a vacuum chamber ina dry etch tool. In one embodiment, the gas is Octafluorocyclopentene(C₅F₈). In certain cases, the gas may have Bromine (Br) or Chlorine (Cl)partially or entirely substituted for the Fluorine. As desired, one ormore other gases, such as Oxygen (O₂), Hydrogen (H₂), Nitrogen (N₂),Helium (He), or Argon (Ar), may be included in the chamber for part orall of the surface treatment of the absorber layer 1400.

[0026] The flowrate of each of the one or more gases present may be inthe range of 1-125 standard cubic feet per minute (sccm). The treatmenttime may be in the range of 10-60 seconds (sec) depending on the gasflowrate and the plasma power selected.

[0027] A Unity II oxide etcher from Tokyo Electron (TEL) may be used toform the top layer 1500 by treating the absorber layer 1400. Sometypical tool and process conditions are described next. The gap may be27 mm. The pressure in the chamber may be about 40 milliTorr. The lowerRF power may be about 1000 Watts. Flowrate may be about 6.0 sccm ofC₅F₈, 3.0 sccm O₂, and 100.0 sccm Argon. The treatment time may be about30 seconds.

[0028] As shown in FIG. 1(e), the combination of top layer 1500,absorber layer 1400, buffer layer 1300, ML mirror 1200, and substrate1100 results in a high inspection contrast EUV mask blank 1700 that hashigh inspection contrast for inspection with UV/DUV light.

[0029] The high inspection contrast EUV mask blank 1700 shown in FIG.1(e) can be further processed to produce a high inspection contrast EUVmask 1800 shown in FIG. 2(d) that has high inspection contrast forinspection with UV/DUV light.

[0030] First, as shown in FIG. 2(a), a high inspection contrast EUV maskblank 1700 is covered with a radiation-sensitive layer, such asphotoresist 1600, that is coated, exposed, and developed with a desiredpattern. The photoresist 1600 has a thickness of about 160-640 nm. Asdesired, a chemically-amplified resist (CAR) may be used. Exposure isperformed with radiation that is appropriate for the photoresist 1600,such as deep ultraviolet (DUV) light or electron beam (e-beam).

[0031] After post-develop measurement of the critical dimension (CD) ofthe features in the pattern in the photoresist 1600, the pattern istransferred into the top layer 1500 and the absorber layer 1400.Reactive ion etch may be used. For example, an absorber layer 1400 maybe dry etched with a gas which contains Chlorine, such as Cl₂ or BCl₃,or with a gas which contains Fluorine, such as NF₃. Argon (Ar) may beused as a carrier gas. In some cases, Oxygen (O₂) may be included. Theetch rate and the etch selectivity depend on power, pressure, andsubstrate temperature.

[0032] The buffer layer 1300 serves as an etch stop layer to helpproduce a good etch profile in the overlying absorber layer 1400. Thebuffer layer 1300 also protects the underlying ML mirror 1200 fromdamage during the etch of the overlying absorber layer 1400.

[0033] Removal of the photoresist 1600 is followed by post-etchmeasurement of the CD of the features in the pattern in the top layer1500 and the absorber layer 1400. Then defect inspection is done,typically with UV/DUV light at a wavelength of about 150-500 nm. Thedefect inspection is based on a comparison of the light signals in thepatterned regions relative to the non-patterned regions. The presentinvention improves contrast by making the top layer 1500 appearsignificantly darker than the ML mirror 1200 in the UV/DUV light. Defectinspection may be done on a microscope such as a Zeiss Axiotron DUVmicroscope. The inspection may be performed at a wavelength of 248 nm or193 nm with a Numerical Aperture (NA) of 0.90-0.95, an eyepiecemagnification of 10×, and an objective magnification of 100-150×.

[0034] During production, masks may be inspected using automated tools.A variety of light sources, including lasers, may be used to provideUV/DUV wavelengths. Typical wavelengths include, but are not limited to,488 nm, 365 nm, 266 nm, 257 nm, 248 nm, 198 nm, and 193 nm. The shorterwavelengths provide better resolution and are necessary as the featureson the mask become smaller. The inspection may be based on die-to-die ordie-to-data. The mask inspection tools may combine optical techniqueswith scanning of the mask to acquire images. If desired, the inspectionmay evaluate phase as well as amplitude.

[0035] As shown in FIG. 2(b), defects may occur in the top layer 1500and the absorber layer 1400 as a result of the pattern transfer from thephotoresist 1600. A first type of defect is a clear defect 1710 while asecond type of defect is an opaque defect 1720. In a clear defect 1710,the absorber layer 1400 should be present, but it is entirely orpartially missing. In an opaque defect 1720, the absorber layer 1400should be removed, but it is entirely or partially present.

[0036] Repair of defects in the top layer 1500 and the absorber layer1400 is performed with a focused ion beam (FIB) tool as needed. A cleardefect 1710 is filled in with an opaque repair material 1730. An opaquedefect 1720 is removed, leaving a Gallium stain 1740 in the underlyingbuffer layer 1300. Thus, the buffer layer 1300 also protects theunderlying ML mirror 1200 from damage during repair of the top layer1500 and the absorber layer 1400.

[0037] The buffer layer 1300 increases light absorption over the MLmirror 1200 when the high inspection contrast EUV mask 1800 is usedduring exposure of photoresist on a wafer. The resulting reduction incontrast can slightly degrade CD control of the features printed in thephotoresist on a wafer so the buffer layer 1300 is removed wherever itis not covered by the top layer 1500 and the absorber layer 1400.

[0038] The buffer layer 1300 may be removed by dry etch or wet etch or acombination of dry etch and wet etch. The dry etch or wet etch used toremove the buffer layer 1300 must not damage the top layer 1500, theabsorber layer 1400, or the ML mirror 1200. The buffer layer 1300 may bedry etched with a gas which contains Fluorine, such as CF₄ or C₄F₈.Oxygen (O₂) and a carrier gas, such as Argon (Ar), may be included. Thebuffer layer 1300 may also be wet etched, especially if it is very thinsince any undercut of the absorber layer 1400 would then be very small.For example, a buffer layer 1300 formed from Silicon Dioxide (SiO₂) maybe etched with an aqueous solution of about 3-5% hydrofluoric (HF) acid.

[0039] The result of the process described above is a high inspectioncontrast EUV mask 1800 having a reflective region 1750 and a dark region1760, as shown in FIG. 2(d). For example, the presence of the top layer1500 over the absorber layer 1400 can reduce reflectivity of a highinspection contrast EUV mask 1800 at UV/DUV wavelengths from greaterthan about 35% to less than about 5%. The corresponding contrast of thehigh inspection contrast EUV mask 1800 at UV/DUV wavelengths can beincreased from less than about 35% to greater than about 80%.

[0040] Another embodiment of the present invention is a high inspectioncontrast EUV mask 2700 as shown in FIG. 3. A high inspection contrastEUV mask 2700 includes a top layer 2500, an absorber layer 2400, abuffer layer 2300, an ML mirror 2200, and a substrate 2100. The highinspection contrast EUV mask 2700 has a first region 2750 and a secondregion 2760. The first region 2750 is reflective because the ML mirror2200 is uncovered. The second region 2760 is darker due to the top layer2500 and the absorber layer 2400.

[0041] First, the high inspection contrast EUV mask 2700 of the presentinvention includes a substrate 2100 with a low defect level and a smoothsurface. It is desirable that the substrate 2100 have a low coefficientof thermal expansion (CTE). The substrate 2100 may be a low CTE glass ora low CTE glass-ceramic. However, in certain cases, the substrate 2100may be Silicon. Although Silicon has a large CTE that may result inundesirable displacement of printed images, Silicon also has a highthermal conductivity and thus is a viable substrate as long as heat canbe removed efficiently from the mask during exposure.

[0042] Second, a multilayer (ML) mirror 2200 is disposed over thesubstrate 2100. The ML mirror 2200 has about 20-80 pairs of alternatinglayers of a high index of refraction material 2210 and a low index ofrefraction material 2220.

[0043] In one embodiment, the ML mirror 2200 has 40 pairs of the highindex of refraction material 2210 and the low index of refractionmaterial 2220. The high index of refraction material 2210 may be about2.8 nm thick Molybdenum (Mo) while the low index of refraction material2220 may be about 4.1 nm thick Silicon (Si). The ML mirror 2200 canachieve about 60-75% reflectivity at the central illumination wavelengthof about 13.4 nm.

[0044] Third, a buffer layer 2300 is disposed over the ML mirror 2200.The buffer layer 2300 is about 20-105 nm thick. The buffer layer 2300provides protection from damage for the underlying ML mirror 2200 duringetch of the absorber layer 2400. The buffer layer 2300 also providesprotection from damage for the underlying ML mirror 2200 during repairof the top layer 2500 and the absorber layer 2400.

[0045] The buffer layer 2300 may be Silicon Dioxide (SiO₂), such as lowtemperature oxide (LTO). Other materials, such as Silicon Oxynitride(SiO_(x)N_(y)) or Carbon (C) may also be used for the buffer layer 2300.

[0046] Fourth, an absorber layer 2400 is disposed over the buffer layer2300. The absorber layer 2400 may be about 45-215 nm of a material thatwill attenuate EUV light, remain stable during exposure to EUV light,and be compatible with the mask fabrication process.

[0047] The absorber layer 2400 may include one or more metals, alloys,and ceramics. Ceramics are compounds formed from metals and nonmetals.Examples of metals include Aluminum (Al), Aluminum-Copper (AlCu),Chromium (Cr), Nickel (Ni), Niobium (Nb), Tantalum (Ta), Titanium (Ti),and Tungsten (W). In some cases, the absorber layer 2400 may partiallyor entirely include borides, carbides, hydrides, nitrides, oxides, orsuicides of various metals. Examples include Nickel Silicide (NiSi),Tantalum Boride (TaB), Tantalum Nitride (TaN), Tantalum Silicide (TaSi),Tantalum Silicon Nitride (TaSiN), and Titanium Nitride (TiN).

[0048] Fifth, a top layer 2500 is disposed over the absorber layer 2400.The top layer 2500 is usually thinner than the absorber layer 2400. Forexample, the top layer 2500 may be about 20 nm thick.

[0049] In one embodiment, the top layer 2500 has higher absorbance thanthe absorber layer 2400. In another embodiment, the top layer 2500 haslower reflectivity than the absorber layer 2400. In still anotherembodiment, the top layer 2500 has both higher absorbance and lowerreflectivity than the absorber layer 2400. The top layer 2500 mayinclude one or more metals, such as Tantalum, and one or more nonmetals,such as Fluorine (F), Oxygen (O), Argon (Ar), Carbon (C), Hydrogen (H),and Nitrogen (N).

[0050] For example, during the inspection of an EUV mask at UV/DUVwavelengths, the presence of a top layer 2500 over an absorber layer2400 can reduce reflectivity from greater than about 35% to less thanabout 5%. For comparison, the ML mirror 2200 has a reflectivity of about60-75%. As a result, the contrast during inspection can be increasedfrom less than about 35% to greater than about 80%. The extent ofbenefit on inspection from using the top layer 2500 will vary dependingon the type, the thickness, and the surface roughness of the one or morematerials selected for the top layer 2500. The extent of benefit oninspection will also depend on the wavelength of the light used forinspection.

[0051] The top layer 2500 may have a rough surface. The roughness mayhelp reduce the interference during exposure between the EUV lightincident on the mask and the EUV light reflected from the top layer2500. The extent of benefit on critical dimension (CD) control fromusing the top layer 2500 will vary depending on the type, the thickness,and the surface roughness of the one or more materials selected for thetop layer 2500. The extent of benefit on CD control will also depend onthe wavelength of the light used for exposure and the step heightbetween the ML mirror 2200 and the top layer 2500.

[0052] Many embodiments and numerous details have been set forth abovein order to provide a thorough understanding of the present invention.One skilled in the art will appreciate that many of the features in oneembodiment are equally applicable to other embodiments. One skilled inthe art will also appreciate the ability to make various equivalentsubstitutions for those specific materials, processes, dimensions,concentrations, etc. described herein. It is to be understood that thedetailed description of the present invention should be taken asillustrative and not limiting, wherein the scope of the presentinvention should be determined by the claims that follow.

[0053] Thus, we have described an EUV mask with high contrast at theinspection wavelength and a process for fabricating such a highinspection contrast EUV mask.

We claim:
 1. A method of fabricating a mask comprising: providing asubstrate; forming a mirror over said substrate, said mirror beingreflective at a first wavelength; forming an absorber layer over saidmirror, said absorber layer being absorbent at said first wavelength;forming a top layer over said absorber layer, said top layer beingabsorbent at a second wavelength; patterning said mask into a firstregion and a second region; and removing said top layer and saidabsorber layer in said first region.
 2. The method of claim 1 whereinsaid first wavelength comprises Extreme Ultraviolet (EUV) wavelength. 3.The method of claim 1 wherein said first wavelength is about 13.4 nm. 4.The method of claim 1 wherein said second wavelength comprisesUltraviolet (UV) or Deep Ultraviolet (DUV) wavelength.
 5. The method ofclaim 1 wherein said second wavelength is about 150-500 nm.
 6. A methodof fabricating a mask comprising: providing a substrate; forming amultilayer mirror over said substrate, said multilayer mirror to reflectat an exposure wavelength; forming an absorber layer over saidmultilayer mirror, said absorber layer to absorb at said exposurewavelength; forming a top layer over said absorber layer, said top layerto increase contrast at an inspection wavelength; patterning said maskinto a first region and a second region; and removing said top layer andsaid absorber layer in said first region.
 7. The method of claim 6wherein said exposure wavelength is Extreme Ultraviolet (EUV)wavelength.
 8. The method of claim 6 wherein said exposure wavelength isabout 13.4 nm.
 9. The method of claim 6 wherein said inspectionwavelength is Deep Ultraviolet (DUV) wavelength.
 10. The method of claim6 wherein said inspection wavelength is about 248 nm.
 11. A maskcomprising: a substrate; a multilayer mirror disposed over saidsubstrate, said multilayer mirror having a first region and a secondregion; an absorber layer disposed over said second region of saidmultilayer mirror, said absorber layer being absorbent at a firstwavelength; and a top layer disposed over said absorber layer, said toplayer being absorbent at a second wavelength.
 12. The mask of claim 11wherein said substrate has a low coefficient of thermal expansion. 13.The mask of claim 11 wherein said first wavelength comprises ExtremeUltraviolet (EUV) wavelength.
 14. The mask of claim 11 wherein saidsecond wavelength comprises Ultraviolet (UV) or Deep Ultraviolet (DUV)wavelength.
 15. The mask of claim 11 wherein said multilayer mirrorcomprises alternating layers of a high index of refraction material anda low index of refraction material.
 16. The mask of claim 11 whereinsaid multilayer mirror comprises Molybdenum and Silicon.
 17. The mask ofclaim 11 wherein a buffer layer is further disposed over said multilayermirror and below said absorber layer in said second region.
 18. The maskof claim 11 wherein said absorber layer comprises a metal.
 19. The maskof claim 11 wherein said absorber layer comprises an alloy.
 20. The maskof claim 11 wherein said absorber layer comprises a ceramic.
 21. Themask of claim 11 wherein said top layer comprises a metal.
 22. The maskof claim 11 wherein said top layer comprises an alloy.
 23. The mask ofclaim 11 wherein said top layer comprises a ceramic.
 24. The mask ofclaim 11 wherein said top layer comprises Fluorine.
 25. The mask ofclaim 11 wherein said top layer comprises Oxygen.
 26. The mask of claim11 wherein said top layer comprises Argon.
 27. The mask of claim 11wherein said top layer comprises Carbon.
 28. The mask of claim 11wherein said top layer comprises Hydrogen.